1. Field of the Invention
This invention relates to the field of computer memories, and more particularly, to an improved method and apparatus for refreshing dynamic random access memories (DRAMs) utilized in a data processing system.
2. Art Background
Data processing systems typically utilize dynamic memories to store and retrieve data. Dynamic memories advantageously provide substantial memory capacity and low power consumption. Dynamic memories, however, must be periodically refreshed in order to preserve the data stored in the memory cells of the dynamic memory. Periodic refreshment is required because a dynamic memory cell typically consists of a single transistor and a capacitor, and the state of the memory cell is determined by the state of the charge on the capacitor, a charge which eventually degrades or leaks off the capacitor.
A dynamic random access memory ("DRAM") defines as part of its specification a refresh period indicating how frequently the DRAM must be refreshed. Such refresh periods typically range between 2 mS and 32 mS. This period of time corresponds, with some worst-case approximation, to the maximum amount of time allowed between refresh operations. A data processing system utilizing a DRAM typically employs circuitry and logic external to the DRAM to insure that these refresh operations are conducted in a timely fashion. One disadvantage of these mandatory refresh operations is that they consume valuable system time, as other operations with respect to the DRAM are necessarily stalled until the refresh operation is completed. For example, if the mandatory refresh operation takes eight clock cycles to complete, all other memory operations with respect to the DRAM are effectively stalled for these eight dock cycles. These eight clock cycles, which might potentially be used for other memory operations, are essentially lost to the mandatory refresh operation.
As will be described, the present invention provides for a data processing system with an optional refresh capability in addition to a mandatory refresh capability. With both an optional refresh capability and a mandatory refresh capability, the present invention provides for the selective use of optional refreshes to minimize the system time lost to refresh operations.